Thermoplastic compounds, unlike wood, metal, or glass, do not rot, rust, or shatter. For that reason, the world in the past seventy years has seen a revolution in material science arising from the combination of a thermoplastic resin and one or more functional additives to provide specific properties to the resin.
Unlike wood but like metal and glass, at a given temperature, a thermoplastic resin can melt. Its processing versatility benefits from its capacity to mix with the functional additives while in a molten state.
But in use, the exposure of a fully formed thermoplastic article to excessive heat or flame can be quite detrimental to property and person.
Flame retardants, drip suppressants, mineral fillers, and char formers are functional additives which can be used to help the thermoplastic compound to retard the effects of heat or flame from melting or even burning. Flame retardant thermoplastic compounds are particular needed when the plastic article is used in any confined space where persons might be present during any condition, planned or emergency, which might expose the plastic article to such excessive heat or flame.
Non-halogenated flame retardants have recently become popular because they minimize the release of halogenated chemicals if the plastic article would begin to degrade, melt, or burn.
To reduce the possibility of the plastic article dripping as it melts, polytetrafluoroethylene (PTFE) is often used as a drip suppressant. But PTFE, even in very small amounts, is a halogenated material, albeit fluorinated rather than chlorinated.
Unfortunately, plasticization is a very common phenomenon in plastics mixed with low molecular weight species, such as plasticizers and low molecular weight flame retardants. It tends to change thermal and mechanical properties of the given polymer resin and hence the compound in which the polymer resin is the main ingredient, such as lowering rigidity at room temperature, lowering heat distortion temperature, increasing elongation break and toughness. (Edmund H. Immergut and Herman F. Mark, Plasticization and Plasticizer Processes, Chapter 1, AMERICAN CHEMICAL SOCIETY, 1965).
Although plasticizers are widely used in the polymer industry using poly(vinyl chloride) resins and compounds, in other parts of the polymer industry, plasticization is typically avoided, especially in engineering thermoplastics.
Non-halogen flame retardant additives are generally low molecular weight chemicals, or oligomers, which have low melting points or glass transition temperatures, hence also they tend to reduce the Heat Distortion Temperature (HDT) of polymer systems by up to 30-40° C. depending on the amount of flame retardant additive(s) present. This decrease of HDT will significantly limit some applications of certain non-halogen flame retardant polymer compounds. Additionally, migration of these small molecular organic chemicals within a compound and high volatilization from the compound are also a concern.
Currently some persons skilled in the art are trying to incorporate some fillers into engineered thermoplastic compounds, such as talc, glass fiber etc, to counteract the HDT reduction described above. Other persons are choosing flame retardants with higher glass transition temperatures (Tg) or melting points as countermeasures; some are using inorganic solid salt flame retardants. Yet others are researching the applicability of polymeric non-halogenated flame retardants, such as polyphosphates, polyphosphazenes and polyphosphonates.
Flame dripping is a very critical issue in non-halogen flame retarded engineered thermoplastic compounds, such as those containing polycarbonate and polycarbonate blends. Typically a small amount (˜0.5 weight percent of the total compound) of polytetrafluoroethylene (PTFE) is used for drip suppression.
But PTFE contains fluorine, which has been regarded in the industry as a “non-halogen” when discussing flame retardants, due to the low usage levels and less reactive nature of fluorinated polymers. However, with increasing desire for truly “halogen-free” polymer compound systems and usage of so-called “Green Materials”, a completely non-halogen, non-fluorine-drip suppressant is needed.